The present invention relates to a piezoelectric semiconductor comprising a single crystal composed mainly of ZnO. More particularly, the present invention relates to a piezoelectric semiconductor which can be suitably used, for example, as an acoustoelectric element for ultrasonic transducers.
ZnO provides an n type semiconductor wherein Zn is present in excess, and further has drawn attention as a piezoelectric material because of its crystal structure.
With respect to the production of a single crystal of high-purity ZnO, there is a literature of "Hydrothermal Growth and Stoichiometric Assessment of ZnO Single Crystals of High Purity" (Noboru Sakagami, Feb. 1988, Research Reports of Akita National College of Technology No. 23). This literature describes the growth of ZnO single crystal by a hydrothermal process. In this process, sintered ZnO is placed at the bottom of an apparatus for crystal growth and ZnO seed crystals are placed at the top of the apparatus; then a solvent, i.e. an aqueous alkali solution containing KOH and LiOH (the solvent is hereinafter referred to as alkali solvent) is fed into the apparatus; in this state, the apparatus inside is placed under the conditions of 370.degree.-400.degree. C. and 700-1,000 kg/cm.sup.2 with the temperature of the apparatus bottom kept 10.degree.-15.degree. C. higher than the temperature of the apparatus top, to grow a single crystal of ZnO.
In the above process, since only the alkali solvent is used as asolvent for crystal growth and the environment for crystal growth is a reducing atmosphere, the amount of excess Zn atoms present in the ZnO single crystal formed is over 10 ppm to over 20 ppm and the electrical conductivity of said single crystal is as high as 10.sup.-2 .about.10.degree. 1/.OMEGA..multidot.cm. Hence, it was tried to add H.sub.2 O.sub.2 to make the growth system under a partial pressure of oxygen to produce a ZnO single crystal of higher purity. As a result, it was found that the ZnO single crystal obtained by H.sub.2 O.sub.2 addition has an electrical conductivity of as low as 10.sup.-10 .about.10.sup.-8 1/.OMEGA..multidot.cm. In the case of H.sub.2 O.sub.2 addition, however, since H.sub.2 O.sub.2 is a strong oxidizing agent, the vessel for crystal growth must be made of platinum or the inside of said vessel must be lined with platinum, making the process disadvantageous costwise.
As mentioned above, ZnO has drawn attention as a piezoelectric material. However, no sufficient study has been made on ZnO as an acoustoelectric material and the above-mentioned production of ZnO single crystal is merely for using the ZnO type single crystal as a piezoelectric material.
The present inventor made a study on the application of ZnO single crystal as an acoustoelectric material by examining the absorption coefficient and acoustoelectric voltage (hereinafter referred to as AE voltage). As a result, the present inventor found that a ZnO single crystal produced by a conventional process, having an electrical conductivity of 10.sup.-2 .about.10.degree. 1/.OMEGA..multidot.cm is not suitable as an acoustoelectric material, particularly an acoustoelectric element for ultrasonic transducers and that a ZnO single crystal having a certain electrical conductivity, etc. is suitable for said application.
The present inventor further found that a ZnO single crystal having an electrical conductivity of 10.sup.-2 .about.10.degree. 1/.OMEGA..multidot.cm, which is not suitable as an acoustoelectric element, can be made into a suitable acoustoelectric element by applying thereto a post-treatment using Li.
The present inventor furthermore found that a ZnO single crystal having an electrical conductivity of 10.sup.-10 .about.10.sup.-8 1/.OMEGA..multidot.cm can be made into a very suitable acoustoelectric element by applying thereto a post-treatment using a trivalent metal.
Hence, the objects of the present invention are to provide a piezoelectric semiconductor having properties (e.g. electrical conductivity) suitable for use as an acoustoelectric element, and a process for production of said semiconductor.